As data output apparatuses of word processors, personal computers, facsimile machines and so forth, printers capable of printing desired information such as texts and images on a sheet-type printing medium, e.g., paper, film and the like, are widely utilized.
Although various printing methods are available for such printers, recently an inkjet printing method has particularly attracted attention because of its capability to perform non-contact printing on a printing medium such as paper, ease of color printing, and low noise. Moreover, for a configuration of such printer, in general a serial printing method is widely adopted because of its low cost and ease of downsizing. According to the serial printing method, a printhead discharging ink in accordance with desired printing data is attached to a carriage and printing is performed by reciprocally scanning the carriage in a direction crossing to the conveyance direction of the printing medium (e.g., paper).
The inkjet printing scheme is designed to print on a printing medium by discharging small ink droplets using various kinds of ink discharge methods such as an electrostatic suction scheme implemented by the application of high voltages, a piezoelectric scheme of mechanically vibrating or displacing ink (colored ink) by using piezoelectric elements, and a thermal scheme using the pressure generated when ink forms bubbles as it is heated. This scheme produces little noise during printing, and allows high resolution, high speed printing by using a printhead having ink orifices formed at a high density. Printing apparatuses using such an inkjet printing scheme are in widespread use even in homes. It has therefore become popular that photos taken by digital cameras are printed by inkjet printing apparatuses.
As digital cameras have improved in function and have been able to take pictures with higher definition, inkjet printing apparatuses which output images have been required to print with higher resolution (density). In order to print with high resolution, it is effective to finely arrange small dots on a printing medium by further reducing the size of ink droplets to be discharged or improve the landing accuracy of ink droplets by decreasing the scanning speed.
On the other hand, the user's need to print at high speed is strong. High speed printing may be realized by, for example, increasing the scanning speed of a printhead or increasing the area where printing can be done by one scanning for printing by increasing the number of nozzles. However, as the scanning speed of the printhead increases, it becomes more difficult to land a discharged ink droplet at a desired position, resulting in a deterioration in landing accuracy. In addition, for example, the vibration of the printing apparatus due to the scanning of the printhead becomes stronger, and large noise is made when the printhead is scanned.
As described above, recently, users have strongly demanded to perform high resolution printing and shorten printing time. It is therefore necessary to provide a printing apparatus which can satisfy these two demands. However, since the methods of satisfying these two demands are contradictory to each other, it is difficult to satisfy the two demands at the same time. The market, however, is demanding an inkjet printing apparatus which satisfies these two demands and suppresses vibration and noise.
An inkjet printing apparatus is designed to print by discharging liquid ink toward a printing medium through small holes (nozzles) formed in the printhead. It is, however, known that since ink is a liquid, when the nozzles are exposed to the atmosphere, the ink in the nozzles increases in viscosity and solidifies.
As ink increases in viscosity and solidifies, a discharge failure, e.g., the occurrence of the landing position offset of an ink droplet or a non-discharge state in which no ink droplet is discharged, occurs, resulting in a deterioration in the quality of a printed image. In order to prevent this, the inkjet printing apparatus has a recovery mechanism for setting the apparatus in a good discharge state. This mechanism performs suction recovery operation of producing negative pressure in the printhead by suction or pressurization, thereby discharging ink in the printhead, or preliminary discharge operation of discharging ink irrespective of printing. Such recovery operation is performed when a predetermined period of time has elapsed while the nozzles are exposed to the atmosphere. In this operation, ink which has increased in viscosity and solidified is discharged outside the nozzles.
Controlling the intervals of this preliminary discharge makes it possible to properly discharge ink under any conditions. In this case, the predetermined period of time is arbitrarily set in accordance with ink, a printhead, and an inkjet printing apparatus. If, for example, the mass (volume) of ink droplets to be discharged is small, since the kinetic energy applied to ink to discharge it is small, the ink cannot be discharged once it increases in viscosity. For this reason, ink whose viscosity has increased lightly must be discharged by preliminary discharge while the evaporation of the ink is small in amount. Therefore, as the mass of ink droplets decreases, the preliminary discharge intervals must be shortened.
In general, preliminary discharge is performed at a predetermined position, e.g., a cap which is provided near the home position and also used for suction recovery or a preliminary discharge port provided on the opposite side of the printing area to the home position (for example, Japanese Patent Laid-Open No. 10-278299).
Caps for suction recovery are indispensable for inkjet printing apparatuses, and hence any printing apparatuses have them. In contrast, a preliminary discharge port is preferably provided with a member which absorbs ink, and is not used for anything other than preliminary discharge, and hence some printing apparatus is designed without a preliminary discharge port in consideration of the cost and space required for the printing apparatus. With this arrangement, preliminary discharge is performed only at the preliminary discharge port.
In addition, since a cap has a mechanism for suction recovery, ink (including pigment ink) discharged by preliminary discharge can be discharged outside the cap. In contrast, a preliminary discharge port often has no suction mechanism. If, therefore, pigment ink is preliminarily discharged through the preliminary discharge port, the pigment ink solidifies into stalactite-like clusters and is deposited. As the degree of deposition increases, the deposit comes into contact with the discharge surface of the printhead or the like. This damages the discharge surface or the like.
For the above reason, a printing apparatus which discharges pigment ink is designed to mainly perform preliminary discharge only at the cap.
As described above, it is very difficult to satisfy both the demands for high speed printing and high resolution printing. In order to realize high resolution, for example, the mass of ink to be discharged needs to be minimized, and the landing accuracy of ink droplets (dots) needs to be increased.
If, however, the mass of ink to be discharged is decreased, a discharge failure tends to occur due to an increase in viscosity of ink and its solidification described above, and the lading accuracy tends to deteriorate because of small kinetic energy and the like. In order to satisfy both the demands for high speed printing and high resolution printing while solving these problems, different printing operations are performed in accordance with different printing modes to achieve the respective purposes.
For example, the printing mode for plain paper on which document data such as a text is to be mainly printed is set to a high speed mode giving priority to printing speed, and the printing mode for glossy paper on which image data such as a photo is to be printed is set to a high resolution mode. In the high speed mode, printing in each printing area is completed by one scanning operation. In the high resolution mode, a method of performing control to realize multi-pass printing is known, in which data to be printed by one scanning operation is decimated, and printing in each printing area is completed by a plurality of scanning operations (for example, Japanese Patent Laid-Open No. 08-290562). The method of performing multi-pass printing while decimating print data in accordance with the printing mode is easier to control than the method of changing the scanning speed, and hence is generally used.
In order to further improve the printing quality, however, it is insufficient to simply perform multi-pass printing. It is also necessary to decrease the scanning speed. Consider a case in which the scanning speed is decreased.
Decreasing the speed of a carriage will increase the intervals at which preliminary discharge can be executed. This method is therefore difficult to apply to an inkjet printing apparatus which uses pigment ink. This is because, in the printing apparatus using pigment ink, preliminary discharge is performed only at the cap near the home position as described above, but the shortest time during which preliminary discharge can be executed cannot be set to be less than the time required for the printhead to reciprocate once.
For example, in the normal printing mode, it takes 0.75 sec for the printhead to reciprocate once. If the preliminary discharge intervals required for the printhead are 3 sec, preliminary discharge is performed once per three reciprocations. Likewise, if the preliminary discharge intervals required for the printhead are 1.3 sec, preliminary discharge must be done every time the printhead reciprocates once.
In contrast to this, if the preliminary discharge intervals required for the printhead are 1.3 sec, the printhead needs to be scanned at a speed that allows the printhead to reciprocate once within 1.3 sec.
In the high resolution mode, if the scanning speed is decreased, the time required for the printhead to reciprocate once is prolonged, and may exceed the preliminary discharge intervals required for the printhead. Assume that it takes 0.75 sec for the printhead to reciprocate once in the normal printing mode, and the preliminary discharge intervals required for the printhead are 1.3 sec. In this case, if the scanning speed in the high resolution mode is decreased to ½ that in the normal printing mode, the time required for the carriage to reciprocate once is 1.5 sec, which is longer than the preliminary discharge intervals required for the printhead.
The prolongation of the shortest time during which preliminary discharge can be executed is also disadvantageous when a printhead which discharges ink droplets with a small mass is used. This is because, as described above, as the mass of ink droplets decreases, kinetic energy decreases, and ink which has increased in viscosity upon evaporation tends to adhere to the discharge surface, resulting in the need to shorten the preliminary discharge intervals.
As described above, as the scanning speed is decreased in performing high resolution printing, if the mass of ink droplets is small, it is difficult to execute preliminary discharge at intervals within necessary intervals.